Testing of objects to detect surface flaws or defects is now known in connection with diverse units, as is testing for conditions leading to, or causing, contamination of surfaces.
Contamination control is important, for example, in integrated circuit fabrication, as is a determination of defects or contaminated areas in wafer substrates. While control of contaminated areas in an overall object can often be readily achieved by eliminating the contaminated areas, many objects do not lend themselves to such readily achieved correction, and this is particularly true, for example, in the case of flat panel displays where cutting out contaminated areas from the entire display is normally impossible. It has therefore become increasingly necessary to be able to determine the presence of defects or surface contamination, particularly where such defects are present at a surface of an object, for example, at the front, or quality, surface of sheets of flat glass.
Detection of defects at a surface is known utilizing a laser to illuminate the surface (see, for example, U.S. Pat. No. 4,893,932), as is the use of a charge coupled device (CCD) to detect sub-micron particles causing scattering of light from a laser (see, for example, U.S. Pat. No. 5,282,151).
In the case of flat glass displays, however, glass sheet inspection now normally involves visual examination under controlled lighting conditions with defect qualification done either by comparing the sample to a uniform standard or by determining the threshold of detection by varying the illumination level. Thus, current inspection has the inherent subjectivity of human inspection as well as requiring significant product handling, and, if quantitative data is desired for statistical quality control, the location of defects and preservation of such data must often be additionally obtained and preserved.
Automated inspection of flat panel display glass has met limited success, due, at least in part, to long inspection times, lack of size resolution, and susceptibility to optical noise from both the environment and the substrate itself.
It is therefore evident that improved systems and methods are needed for determining defects at the surface of objects, and, particularly, in determining defects at a surface of a sheet of flat glass.